The present invention relates to a method for restoring a subterranean formation which may have become contaminated during an in situ leach operation and more particularly relates to a method of removing contaminants from a subterranean clay-containing formation after an in situ leach operation to restore the purity of any ground waters that may be present in the formation.
In a typical in situ leach operation, wells are completed into a mineral or metal value bearing (e.g., uranium) formation and a lixiviant is flowed between wells to dissolve the desired values into the lixiviant. The pregnant lixiviant is produced to the surface where it is treated to recover the desired values from the lixiviant. Unfortunately, many known, highly effective lixiviants not only leach the desired values from the formation but, also, they react with certain formations to give up chemical substances which remain in the formation after the lixiviants pass therethrough. Where the formation also contains ground waters and/or a water source which would otherwise be fit for human and/or animal consumption, these chemical substances will likely create a substantial contamination problem for this water. If this be the case, the formation must be treated after a leach operation to remove these contaminants to restore the purity of the water.
One method for improving the purity of a contaminated water source is to merely pump the water from the formation until the contaminant reaches an acceptably low level. Another simple method is to pump uncontaminated water through the formation to flush out the contaminants. These methods work well where the contaminants are soluble and are not absorbed by some component of the formation from which it can only be released at a very slow rate. If the contaminants are absorbed by the formation, extremely large volumes of water must be used to adequately restore the formation.
In many known uranium and related value bearing formations, a substantial part of the formation matrix is comprised of swelling type clays (e.g., smectite). This type formation presents a real formation water contamination problem when a known, highly effective lixiviant comprised of an aqueous solution of ammonium carbonate and/or bicarbonate is used to leach the desired values from the formation. Here, the ammonium ions from the lixiviant are strongly absorbed by the smectite clays in the formation which make their removal by flushing with ground water a very slow and extended process.
One method for removing ammonium ions from a formation following a uranium leach operation is disclosed in copending U.S. application Ser. No. 781,242, filed Mar. 25, 1977, now U.S. Pat. No. 4,079,783, wherein an aqueous solution of a strong, soluble, alkaline compound is flowed through the formation to convert ammonium ions to an un-ionized form, i.e., ammonia (NH.sub.3), which can easily be flushed from the formation. However, in certain instances, some of the more desirable alkaline solutions, e.g., sodium hydroxide, have a tendency to cause the clay in the formation to swell, which substantially decreases the permeability of the formation. If this swelling becomes severe enough, the restoration operation may have to be terminated before sufficient ammonium ions have been removed to adequately restore the formation.